Neurologists use the term paresis to describe weakness, and plegia to describe paralysis in which all voluntary movement is lost. The term paresis comes from the πάρεσις "letting go" or "paralysis" from παρίημι "to let go, to let fall."

Paralysis is most often caused by damage to the nervous system or brain, especially the spinal cord. Paralysis often includes loss of feeling in the affected area.

Paralysis may be localized, or generalized, or it may follow a certain pattern. For example, localized paralysis occurs in Bell's palsy where one side of the face may be paralysed due to inflammation of the facial nerve on that side. Patients with stroke may be weak throughout their body (global paralysis) or have hemiplegia (weakness on one side of the body) or other patterns of paralysis depending on the area of damage in the brain. Other patterns of paralysis arise due to different lesions and their sequelae. For example, lower spinal cord damage from a severe back injury may result in paraplegia, while an injury higher up on the spinal cord, such as a neck injury, can cause quadriplegia. Patients with paraplegia or quadriplegia often use equipment such as a wheelchair or standing frame for mobility and to regain some independence.

Most paralyses caused by nervous system damage are constant in nature; however, there are forms of periodic paralysis, including sleep paralysis, which are caused by other factors.

Paralysis can occur in localised or generalised forms, or it may follow a certain pattern. Most paralyses caused by nervous-system damage (i.e. spinal-cord injuries) are constant in nature; however, some forms of periodic paralysis, including sleep paralysis, are caused by other factors.

Ascending paralysis presents in the lower limbs before the upper limbs. It can be associated with:

Some species of wasp, to complete the reproductive cycle, the female wasp paralyses a prey item such as a grasshopper and places it in her nest. She then lays eggs in the paralysed insect, which is devoured by the larvae when they hatch.

A well-known example is the tetrodotoxin of fish species such as Takifugu rubripes, the famously lethal pufferfish of Japanese fugu. This toxin works by binding to sodium channels in nerve cells, preventing the cells' proper function. A non-lethal dose of this toxin results in temporary paralysis. This toxin is also present in many other species ranging from toads to nemerteans.

Paralysis can be seen in breeds of dogs that are chondrodysplastic. These dogs have short legs, and may also have short muzzles. Their intervertebral disc material can calcify and become more brittle. In such cases, the disc may rupture, with disc material ending up in the spinal canal, or rupturing more laterally to press on spinal nerves. A minor rupture may only result in paresis, but a major rupture can cause enough damage to cut off circulation. If no signs of pain can be elicited, surgery should be performed within 24 hours of the incident, to remove the disc material and relieve pressure on the spinal cord. After 24 hours, the chance of recovery declines rapidly, since with continued pressure, the spinal cord tissue deteriorates and dies.

Another type of paralysis is caused by a fibrocartilaginous embolism. This is a microscopic piece of disc material that breaks off and becomes lodged in a spinal artery. Nerves served by the artery will die when deprived of blood.

The German Shepherd is especially prone to developing degenerative myelopathy. This is a deterioration of nerves in the spinal cord, starting in the posterior part of the cord. Dogs so affected will become gradually weaker in the hind legs as nerves die off. Eventually their hind legs become useless. They often also exhibit faecal and urinary incontinence. As the disease progresses, the paresis and paralysis gradually move forward. This disease also affects other large breeds of dogs. It is suspected to be an autoimmune problem.

Cats with a heart murmur may develop blood clots that travel through arteries. If a clot is large enough to block one or both femoral arteries, there may be hind leg paralysis because the major source of blood flow to the hind leg is blocked.

Many snakes exhibit powerful neurotoxins that can cause non-permanent paralysis or death.